What do implementers need in order to ship Temporal?

[sffc]

This proposal is Stage 2. Temporal is Stage 3. At least one browser is close to shipping Temporal. However, this proposal is lagging behind.

We still have fundamental issues that are not fully resolved: for example, what era codes to use, and how calendars with leap months should behave in arithmetic.

It seems like we should specify these things and get this proposal to Stage 3 before shipping Temporal implementations that could break after we make these changes. Right?

CC @anba

[anba]

The Firefox implementation uses the era codes and limits which are currently used in this proposal. Except for ethiopic, where the ICU4X era limits are used. Also see tc39/proposal-temporal#2900 (comment) and tc39/proposal-temporal#2900 (comment) which has examples where CE/BCE-style year counting is used for ethiopic.

Arithmetic for calendars with leap months is supported → https://searchfox.org/mozilla-central/rev/9c395b6371eaea0d15f9c8a37889022be350cf0b/js/src/builtin/temporal/Calendar.cpp#3489-4142.

Chinese/Dangun calendar years could also lead to possible incompatibilities across implementations. Maybe this is also worth specifying in more detail. Cf. https://bugzilla.mozilla.org/show_bug.cgi?id=1941372.